Fix buggy completion of ~/some in vi mode (#41)

This commit fixes the bug reported in:
https://github.com/att/ast/issues/682
The following sequence fails in vi mode because ksh looks in the
wrong part of the 'virtual' buffer:

$ touch ~/testfile
$ ls ~/test<tab>

The fix is to change 'virtual[i]' to 'virtual[last_virt]' in the
bugged section of code. The other changes are to make sure listing
files in a directory with something like 'ls /etc/<tab>' calls the
code for Ctrl+L to preserve 'ls /etc/' rather than try (and fail)
to complete the directory name, producing 'ls /etc\n/'. This bugfix
was backported from ksh93v- 2013-10-10-alpha.

src/cmd/ksh93/edit/vi.c
 - Backport the bugfix from ksh93v- 2013-10-10-alpha for this
   problem.

src/cmd/ksh93/tests/pty.sh
 - Add a regression test for this issue using pty, adjusted slightly
   for a fake home directory in /tmp.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-24:
+
+- Fixed buggy tab completion of tilde-expanded paths such as
+  ~/some in 'vi' mode.
+
 2020-06-23:
 
 - Fixed a bug that caused combining process substitution with redirection